The present invention relates to a process for the working-up, by thermal pressure decomposition (TPD), of alkaline waste waters formed in the washing of crude nitrobenzene.
In addition to water, the alkaline waste waters from the washing of crude nitrobenzene usually contain residual amounts of benzene and nitrobenzene, as well as nitrohydroxyaromatics. The following nitrohydroxyaromatics, which can also be present in the form of their water-soluble salts, may be mentioned as examples: mono-, di- and trinitrophenols, mono-, di- and trinitrocresols, mono-, di- and trinitroresorcinols and mono-, di- and trixylenols. Possible salt-forming agents are any metals that are capable of forming water-soluble salts with the nitrohydroxyaromatics. The alkali metals, e.g. lithium, sodium, potassium and rubidium, may preferably be mentioned.
The basic TPD process for the treatment of waste waters containing aromatic nitro compounds is described in EP 0 005 203 A2 and EP 0 503 387 A1. EP 0 005 203 A2 describes a process for the working-up of waste waters containing nitrohydroxyaromatics wherein the waste waters are treated at a pressure of 50-250 bar and a temperature of 150-500° C. with the exclusion of air and oxygen.
EP 0 503 387 A1 describes a similar process, but in this case the alkaline waste water is worked up by the addition of nitric acid and subsequent treatment in temperature ranges of 180-350° C. and a pressure range of 40-250 bar.
However, both processes have considerable disadvantages.
EP 0 005 203 A2 does not describe the removal of organic hydrocarbons, such as benzene or nitrobenzene, which are obtained in an adiabatic nitration process corresponding to the state of the art. The purification of the waste water according to the teaching of EP 0 005 203 A2 is therefore inadequate and the consumption of sodium hydroxide solution in the TPD becomes very high.
In EP 0 503 387 A1, the decomposition of nitrobenzene is not complete, so a further treatment of the waste water is necessary. Moreover, the nitrobenzene contained in the waste water is decomposed in the TPD, thereby reducing the yield achieved. The presence of nitric acid in the TPD, which is required according to the teaching of EP 0 503 387 A1, also increases the process costs in several respects: (1) the consumption of nitric acid and (2) the high material burden and the associated high investment costs for a titanium-lined tubular reactor. An additional disadvantage, not mentioned in EP 0 503 387 is the need to neutralize the described alkaline waste water prior to addition of the nitric acid, which can optionally be effected with the appropriate equivalent amount of nitric acid.